In many wireless communication devices such as cellular telephones, wireless-enabled laptops, wireless-enabled personal digital assistant (PDA) devices, and the like, radio transmitters are often situated in close proximity to a human user. There is currently concern that exposure to electromagnetic radiation emitted by such transmitters may pose a health risk to the user or other persons sufficiently close to the transmitter. Undesired electromagnetic radiation may also interfere with operation of nearby electronics or communication devices.
To reduce health risks, regulatory bodies such as the FCC have mandated limits for safe exposure to radio frequency (RF) energy corresponding to electromagnetic radiation of such transmitters. These limits are given in terms of a unit referred to as the Specific Absorption Rate (SAR), which is a measure of the amount of radio frequency energy absorbed by the body when using a wireless communication device. The FCC requires device manufacturers to ensure that their devices comply with these objective limits for safe exposure. The current FCC limit to public exposure from cellular telephones is an SAR level of 1.6 watts of absorbed RF energy per kilogram of body tissue. Some users may seek to reduce their exposure to RF energy even below the FCC limit.
Several approaches have been proposed to reduce undesired exposure to RF energy by deflection. For example, U.S. Pat. No. 7,034,772 discloses a flexible metallic tape, shaped around the antenna, for deflecting and blocking antenna radiation from cellular telephones. As another example, United States Patent Application Publication No. 2004/0198264 discloses a telephone radiation shielding apparatus including a conductive sheet and strip to receive incident ionizing and non-ionizing radiation. The shield may be built in to a telephone or provided in a modification kit. As another example, United States Patent Application Publication No. 2002/0137473 discloses a shield apparatus for placement over speaker openings of a cellular phone to obstruct electromagnetic radiation. The shield comprises two layers of metallic mesh and reportedly absorbs the radiation, while the mesh structure allows the passage of sound waves. As another example, U.S. Pat. No. 6,341,217 discloses a portable telephone having an antenna and a grounded metallic surface interposed between antenna and user. The metallic surface is spaced apart from the antenna by one-quarter wavelength to maximize reflection. As another example, U.S. Pat. No. 6,095,820 discloses an antenna assembly including a driven antenna and a shield apparatus which absorbs and redirects radiation outward away from the user.
However, the above approaches may cause undesirable negative effects on device transmitter operation. For example, mutual coupling between the metallic shield and the device antenna may negatively impact antenna operation. These approaches represent broad or untargeted attempts to minimize exposure to electromagnetic radiation which may not be feasible in some cases.
As another approach, United States Patent Application No. 2008/0014872 discloses a tuned passive antenna which captures cellular telephone antenna radiation and converts the captured radiation to electric current, which is dissipated by operating a thermal, mechanical or electrical device, thereby reducing exposure to undesired electromagnetic radiation. However, this approach requires conversion of captured RF energy to a form usable by the device being operated, which may be complicated and inefficient.
Therefore there is a need for a device for reducing exposure to electromagnetic radiation that is not subject to one or more limitations in the prior art.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.